In order to meet the increasing demand on higher capacity and/or lower latency, Long Term Evolution (LTE) communication systems need to continuously evolve. The available frequency band for LTE evolution may be in the range from 10 GHz to 30 GHz. At such high frequency, path loss will be very high and coverage will be limited. Hence, a dense deployment of nodes will be desired. It is quite difficult to deploy fixed backhauls in such scenario. Instead, since the spectrum at such high frequency band is abundant, it will be very cost effective to adopt a self-backhaul scheme in which a backhaul link and an access link use the same frequency.
FIG. 1 shows a simplified example of such self-backhaul scheme. As shown in FIG. 1, a relay 102 (which is a self-backhaul node) is wirelessly connected to a donor evolved NodeB (eNB), which is in turn connected with a Core Network (CN) 106. The link between the relay 102 and the donor eNB 104 is referred to as a backhaul link of the relay 102, or an access link of the donor eNB 104. The relay 102 also serves a User Equipment (UE) 108. The link from the UE 108 to the relay 102 is referred to as an access link of the relay 102, or a backhaul link of the UE 108. The path from the UE 108 to the relay 102 and then to the donor eNB 104 can be referred to herein as a wireless backhaul path. On the wireless backhaul path, a direction from the node farthest from the CN 106 (i.e., the UE 108) to the node closest to the CN 106 (i.e., the donor eNB 104) is referred to herein as the upstream direction, and a direction from the node closest to the CN 106 (i.e., the donor eNB 104) to the node farthest from the CN 106 (i.e., the UE 108) is referred to herein as the downstream direction. Each node controls the transmission on its access link (if any) and has the transmission on its backhaul link (if any) controlled by its upstream node. It is assumed here that a Time Division Multiplexing (TDM) scheme is employed between the access link and the backhaul link of the relay 102. In the following, interference scenario related to the relay 102 will be discussed, without loss of generality.
FIG. 2 illustrates an interference scenario related to the relay 102 of FIG. 1. FIG. 2 shows four consecutive subframe (SF) periods #0˜#3. The dashed lines in FIG. 2 denote the reference timing that has been synchronized among the relay 102, the donor eNB 104 and the UE 108. The hatched bars in FIG. 2 denote subframes.
As shown in FIG. 2, at 201, the donor eNB 104 transmits a subframe #0 to the relay 102 within the SF period #0. Due to propagation delay between the donor eNB 104 and the relay 102, at 202, the subframe #0 is received by the relay 102. It can be seen from FIG. 2 that a portion of the received subframe #0 has intruded into the SF period #1. At 203, the relay 102 transmits a subframe #1 to the UE 108 within the SF period #1. Therefore, the portion of the received subframe #0 that has intruded into the SF period #1 overlaps the transmitted subframe #1 and thus suffers interference from the transmission of the subframe #1, as indicated by the arrow between the subframes #0 and #1.
At 204, the UE 108 transmits a subframe #2 to the relay 102. The subframe #2 is transmitted on the relay 102's access link and thus its transmission timing is controlled by the relay 102. According to a Timing Advance (TA) command from the relay 102, the UE 108 advances the transmission of the subframe #2 by an amount of TA1 with respect to the reference timing of the SF period #2, such that the subframe #2 can be received by the relay 102 within the SF period #2 at 205. At 206, the relay 102 transmits a subframe #3 to the donor eNB 104. The subframe #3 is transmitted on the relay 102's backhaul link and thus its transmission timing is controlled by the donor eNB 104. According to a TA command from the donor eNB 104, the relay 102 advances the transmission of the subframe #3 by an amount of TA2 with respect to the reference timing of the SF period #3, such that the subframe #3 can be received by the donor eNB 104 within the SF period #3 at 207. However, it can be seen from FIG. 2 that a portion of the subframe #3 has intruded into the SF period #2. Therefore, the portion of the subframe #3 that has intruded into the SF period #2 overlaps the subframe #2 and thus creates interference on the reception of the subframe #2, as indicated by the arrow between the subframes #2 and #3.
In order to solve such Tx-to-Rx interferences, it has been proposed to postpone the reference timing of the relay 102. FIG. 3 shows an exemplary situation when this proposal is applied to the scenario shown in FIG. 2. In FIG. 3, the dashed lines 310 denote the reference timing of the donor eNB 104 and the solid lines 320 denote the reference timing of the relay 102. Compared with the reference timing 310, the reference timing 320 is postponed by a timing offset that equals to the propagation delay between the relay 102 and the donor eNB 104. In addition to the SF periods #0˜#3 corresponding to the reference timing of the donor eNB 104, FIG. 3 shows SF periods #0′˜#3′ corresponding to the reference timing of the relay 102.
As shown in FIG. 3, at 301, the donor eNB 104 transmits a subframe #0 to the relay 102 within the SF period #0. Due to propagation delay between the donor eNB 104 and the relay 102, at 302, the subframe #0 is received by the relay 102 exactly within the SF period #‘0’. At 303, the relay 102 transmits a subframe #1 to the UE 108 within the SF period #1′. It can be seen from FIG. 3 that the received subframe #0 does not overlap the transmitted subframe #1 and thus no Tx-to-Rx interference occurs. That is, the propagation delay has been absorbed by the timing offset.
At 304, the UE 108 transmits a subframe #2 to the relay 102. The subframe #2 is transmitted on the relay 102's access link and thus its transmission timing is controlled by the relay 102. According to a TA command from the relay 102, the UE 108 advances the transmission of the subframe #2 by an amount of TA1′ with respect to the reference timing of the SF period #2 and the subframe #2 is received by the relay 102 at 305. At 306, the relay 102 transmits a subframe #3 to the donor eNB 104. The subframe #3 is transmitted on the relay 102's backhaul link and thus its transmission timing is controlled by the donor eNB 104. According to a TA command from the donor eNB 104, the relay 102 advances the transmission of the subframe #3 by an amount of TA2 with respect to the reference timing of the SF period #3, such that the subframe #3 can be received by the donor eNB 104 within the SF period #3 at 307. Here, in order to prevent the subframe #2 received at 305 from being interfered by the subframe #3 transmitted at 306, the relay 102 needs to take into account the TA command from the donor eNB 104 in determining the TA command for the UE 108. That is, the relay 102 needs to calculate TA1′ by adding TA2 to TA1, i.e., TA1′=TA1+TA2.
However, when applied to a multi-hop wireless backhaul path, the timing offset scheme of FIG. 3 becomes problematic. First, the nodes along the path will be asynchronous to each other. Some advanced features dependent on synchronization, such as Coordinated Multi-Point (COMP), cannot be applied in this case. Second, as discussed in connection with FIG. 3, at a particular node, propagation delays of all its upstream nodes along the path will be aggregated. Therefore, the upstream transmission timing of that node may be advanced too much when compared to the synchronized reference timing. For example, in this case it may not receive any Physical Random Access Channel (PRACH) message from UEs it serves. Third, a particular node may have its upstream node changed. In this case, the reference timing of that node needs to be re-adjusted due to the changed propagation delay, which may cause confusion for UEs it serves.
In addition to the above Tx-to-Rx interferences, there may also be a problem of Tx-Tx overlap. Referring to FIG. 2 again, recall that at 203 the relay 102 transmits a subframe #1 to the UE 108 within the SF period #1. At 204, instead of the UE 108 transmitting a subframe to the relay 102, it is assumed here that the relay 102 transmits a subframe #2 to the donor eNB 104. In this case, the subframe #2 is transmitted on the relay 102's backhaul link and thus its transmission timing is controlled by the donor eNB 104. According to a TA command from the donor eNB 104, the relay 102 advances the transmission of the subframe #2 by an amount of time with respect to the reference timing of the SF period #2. Hence, as shown, a portion of the subframe #2 has intruded into the SF period #1. Such partial overlap between the subframes #1 and #2, as indicated by the arrow between the subframes #1 and #2, may force the relay 102 to reduce the transmit power of one or both of these two subframes to prevent their combined transmit power from exceeding a predetermined limit. The scheme showed in FIG. 3 cannot solve this problem.
There is thus a need for improved transmission coordination among nodes on a wireless backhaul path.